1. Field of the Invention
This invention relates to an apparatus for detecting and measuring minute defects in sheet or rolled material and, more particularly to an apparatus for detecting and measuring defects in sheet or rolled material which is capable of highly efficient measurement of even the area of minute defects in relatively large-area sheet material or wide-width rolled material.
2. Description of the Prior Art
Pin holes, irregularities and other kinds of defects requiring detection are likely to occur in various types of films as, for example, in the thin films produced by vacuum deposition, sputtering and other methods of producing thin films in vacuo, coated films obtained by the various coating methods, or the blackened portion of a lithographic type photosensitive film after development.
As a specific example of a problem resulting from the existence of such defects there can be mentioned the case of heat mode recording media of the type wherein recording is carried out by using a laser beam or other high energy beam to melt or evaporate a metallic recording layer of high optical density formed by vacuum deposition etc. to convert the irradiated portions of the recording layer to portions of low optical density. As any pin hole or other surface defect present in such a medium will have low optical density, it will be read as a recorded portion in the read-out process even though no information has in fact been recorded on this part of the medium. On the other hand, in the case of magnetic recording medium, the presence of a pin hole not only prevents the recording of information at the position where it exists but may even disrupt the magnetic field of the surrounding portions of the medium, making accurate read-out impossible.
Because of the problems caused by such defects in recording media, a number of inspection methods have been proposed for determining whether or not recording media are suitable for use.
One such inspection method detects surface defects in the medium by using a laser beam to scan the medium in the X and Y directions. However, if this method is to be used not only for the detection of defects but also for the determination of the size and shape of minute defects, it becomes necessary to reduce the diameter of the scanning laser beam to a very small size. This in turn means that the scanning range has to be severely limited in view of the aberration of the scanning lens, the precision of the oscillating (rotating) mirror and other considerations. For example, where the size and shape of minute defects measuring only a few .mu.m in diameter are to be determined, it becomes necessary to reduce the diameter of the laser beam so that the size of the scanning spot on the recording medium is on the order of several tens of .mu.m. As a consequence, where an optical system of the ordinary type is used, the size of the area that can be scanned with the required degree of accuracy is four inches at the very most. Therefore, when the inspection is to be conducted in respect of a vacuum deposited thin film or other material having a relatively large area, it becomes impossible to scan the entire area in one operation and the area must be divided into a number of inspection regions for successive scanning. As the time required for scanning the entire area in this way is considerable, the inspection operation consumes much time and lacks practicability.
On the other hand, if the laser beam is used with no reduction in diameter or if a relatively large diameter beam is used with only moderate reduction, it is of course possible to carry out scanning of a large area without being limited by the precision of the optical system. In this case, however, the detection accuracy will be low and it will not be possible to determine the size or shape of the detected defects.